Six autosomal recessive mutations affecting the CNS, PNS or skeletal muscle of the laboratory mouse have been or are being transferred by repetitive backcrosses to each of two inbred strains: C57BL/6J and 129/J. Interstrain hybrid homozygotes have been shown to possess increased general vigor and greatly increased survival duration. Mutant organisms on the genetic background of either inbred strain or the interstrain hybrid are made available to several laboratories as well as to our own for systematic analysis of the cellular locus of gene action, the sequence of cellular involvement, pliotropic effects and the metabolic deficiencies responsible. Histochemical, neuroanatomical and electrophysiological methods are being utilized. The mutations include: wl-Wabbler-Lethal with retinal ganglion cell and optic nerve atrophy; primary and secondary vestibular degeneration and localized brain stem reticular formation degeneration. dt-Dystonia Musculorum with medulary and pontine reticular formation cellular and axonal degeneration, dorsal root ganglion cell degeneration and primary and secondary stretch- afferent degeneration. In late stages, some lower motor neuron cell death may be present. dtJ- Dysafferent, an allele of dt with similar but milder signs. wr-Wobbler with lower motor neuron degeneration. cg-Ching Luan with peripheral vestibular and cerebellar dysfunction. dy-Dystrophia Muscularis, with a (presumed) primary myopathy of skeletal muscles most severe in the distal extremities.